subtly notify developer when packaged app installation is out-of-date

[mykmelez]

If a developer changes a packaged app and then neglects to update it (i.e. reinstall it in the Simulator), they might be confused when they start the app and don't see their changes.

The Simulator should reduce the likelihood of this happening by monitoring the status of the app's source files (including but not limited to the manifest) and subtly notifying the developer—both in the Dashboard and in the Simulator's interface—when a file has changed more recently than the app has been updated.

It's important that the notification be subtle, because most of the time, the developer won't have neglected to update the app, they'll just not have done so yet. And making the notification insistent will just be annoying.

It's also important that the notification appear in both the Dashboard and the Simulator, since the new Refresh feature means developers will often go back and forth between their editor and the Simulator without visiting the Dashboard.

A notification in the Simulator could be a subtle alert icon in the toolbar when the app is active. A tooltip on the icon would provide more information, while clicking the icon would update the app, making it the equivalent of a Refresh button for out-of-date apps.

Similarly, an alert icon in the app's Dashboard entry could appear under the same circumstances and behave the same way.

[nickdesaulniers]

This has some overlap with #444